1. Field of the Invention
This invention relates to apparatus for the blending of fluid material and particulate material and is more particularly concerned with systems for feeding fluid material and particulate material to blender means in a continuous operation.
2. Description of the Prior Art
Particle boards, and other composites derived by binding together particulate material using an adhesive binder, are prepared by coating the particulate material with the adhesive binder and forming the coated particles into a mat which is then subjected to the action of heat and pressure in order to prepare the final composite. In commercial production processes the coating of the particles, the formation of the mat, and the pressing operation are carried out in a substantially continuous manner; see, for example, U.S. Pat. Nos. 3,796,529 and 4,320,715. Illustratively, the particulate material and the binder are brought together and blended using various types of mechanical blender and then passed to a storage bin or the like. From the latter the coated particles are dispensed on to a moving belt to form a mat which is subsequently conducted on the moving belt through a zone in which the mat is subjected to heat and pressure to form the particle board.
The adhesive used to prepare particle boards has hitherto commonly been a phenol-formaldehyde resin, but, more recently, polyisocyanates, particularly polymethylene polyphenyl polyisocyanates, have been used as the adhesive binder. Various methods of mechanically blending the particulate material and the binder have been described and employed in the art. Since the cost of the binder is a significant proportion of the total cost of the raw materials in production of the particle boards, it is desirable that the mixing of the binder and the particulate material be carried out as efficiently as possible without any significant loss of binder in the process. Centrifugal blenders have been employed in which the binder is dispensed through rotating radial dispensing arms in a housing through which the particulate material is being fed. U.S. Pat. No. 4,320,715 discusses this type of centrifugal blender and notes certain drawbacks. The patent describes a different form of blending in which the particles are caused to fall downwardly on the periphery of a blender vessel with a spray of fluid coating material being directed outwardly against the falling furnish by use of a series of rotating inverted conical atomizer disks.
The above types of systems appear to be reasonably satisfactory when phenol-formaldehyde is employed as the binder resin. However, in the case of the polyisocyanate binders, the quantity of binder being applied to the particulate material is significantly less and cannot be easily dispensed in a uniform manner using the above types of operation. Further, in a particular method of employing polyisocyanates as the binder resin, the polyisocyanate is emulsified in water and the emulsion is applied to the particulate material. Such emulsions of polyisocyanate have only a limited stability and, if prepared and stored prior to a production run, can be rendered useless or unsatisfactory if any breakdown of the production line occurs involving long delays which extend beyond the useful life of the emulsion.
When using polyisocyanates as the binder in coating particulate material, particularly for particle boards, it is highly desirable that the coating operation can be interrupted at any given moment in order to accommodate shutdowns of the production line in which the coated particles are being converted to finished boards. The previous types of blender used with phenol-formaldehyde resin binders are not readily adapted to such interruption in operation. Further, it is desirable, when using the polyisocyanate in the form of an aqueous emulsion, to provide systems which do not require production and storage of the emulsion in a preliminary step, but which permit the emulsion to be formed in situ at the time of dispensing and blending with the particulate material.
It is an object of the present invention to provide systems which meet these requirements. It is a further object of this invention to facilitate operations which utilize polyisocyanate as the binder either in neat form or in the form of an aqueous emulsion. Other advantages which are provided by the systems described below will be apparent to one skilled in the art.